Summary/Abstract Subjects with aspirin-exacerbated respiratory disease (AERD) have severe chronic eosinophilic inflammation in the respiratory mucosa, excessive production of cysteinyl leukotrienes (cys- LTs), and basal activation of mast cells (MCs) with elevated levels of tryptase, histamine, and prostaglandin D2 (PGD2) detected in biologic fluids. In the current funding period, we reported that IL-33 is highly expressed in the nasal polyps of patients with AERD. The murine model of AERD also demonstrates increased lung cysLTs, IL-33, eosinophils, and MC activation in the absence of aspirin (ASA) challenge, as compared with WT controls. Each of these features is markedly attenuated by the targeted deletion of leukotriene C4 synthase (LTC4S), indicating that cysLTs play a proximal role in driving type 2 inflammation and basal MC activation in the AERD model. The continuation of this project focuses on identifying the cysLT receptors (CysLTRs), mechanisms, and cellular targets by which cysLTs drive type 2 inflammation. The central hypothesis is that dysregulated cysLT production and signaling through the type 2 cysLTR (CysLT2R) play a proximal role in eliciting type 2 inflammation and priming MCs in the respiratory mucosa through the generation of IL-33 and the activation of the pulmonary endothelium. The findings from this proposal will have immediate implications for AERD, in which cysLTs are overproduced and which is incompletely treated by antagonism of the type 1 cysLTR, CysLT1R. In Aim 1, we will examine the role of CysLT2R in driving IL-33 generation and type 2 pulmonary inflammation in two murine models using novel null strains. We will validate the requirement for IL-33 in cysLT-driven type 2 inflammation, and identify the cellular sources of IL- 33 production. In Aim 2, we will use bone marrow chimeras and novel Cysltr2flox/flox strains to identify the relevant CysLT2R-expressing cells that regulate these models. In Aim 3, we will use transcriptional profiling of sorted lung MCs from the murine AERD model to examine the consequences of CysLT2R/IL-33 signaling on priming lung MCs for eicosanoid production and patholobiologic function. We will use the murine MC transcriptome defined in several conditions to understand the transcriptional profile of MCs directly sorted from nasal polyp tissue from adults with AERD and controls (in collaboration with Project 1).